On Fri, 23 Feb 2024, Vlastimil Babka wrote:
On 2/23/24 10:37, Chengming Zhou wrote:
On 2024/2/23 17:24, Vlastimil Babka wrote:
I think this is a better direction! We can use RCU list if slab can be freed by RCU.
Often we remove slab from the partial list for other purposes than freeing -
i.e. to become a cpu (partial) slab, and that can't be handled by a rcu
callback nor can we wait a grace period in such situations.
IMHO, only free_slab() need to use call_rcu() to delay free the slab,
other paths like taking partial slabs from node partial list don't need
to wait for RCU grace period.
All we want is safely lockless iterate over the node partial list, right?
Yes, and for that there's the "list_head slab_list", which is in union with
"struct slab *next" and "int slabs" for the cpu partial list. So if we
remove a slab from the partial list and rewrite the list_head for the
partial list purposes, it will break the lockless iterators, right? We would
have to wait a grace period between unlinking the slab from partial list (so
no new iterators can reach it), and reusing the list_head (so we are sure
the existing iterators stopped looking at our slab).
We could mark the state change (list ownership) in the slab metadata and
then abort the scan if the state mismatches the list.
Maybe there's more advanced rcu tricks but this is my basic understanding
how this works.
This could get tricky but we already do similar things with RCU slabs
objects/metadata where we allow the resuse of the object before the RCU
period expires and there is an understanding that the user of those
objects need to verify the type of object matching expectations when
looking for objects.
